There is a demand for highly conductive metal flake for applications such as electronic shielding and circuitry, and for pigments, e.g. for water-based paints, inks, etc. One technique for producing metal flake involves a wet milling technique in which metal powder is ball milled in a low viscosity liquid, referred to herewith as a mobile liquid, the liquid eventually being driven off, e.g. by centrifuging and vacuum drying. The liquid is recovered and if in satisfactory form therefor it is reused. It is also known to incorporate a small amount of a lubricant such as fatty acids in the liquid medium to minimize or prevent welding of the particles being milled. U.S. Pat. Nos. 3,709,439 and 3,941,584, for example, disclose a method of making stain-resistant metals and alloys including nickel, which have smooth, specularly reflective surfaces, are bright and lustrous and are characterized by aesthetic effectiveness in paint films and other applications. However, the flake produced by the described method is essentially non-conductive or poorly conductive. A further problem in the milling process is the degeneration of the liquid medium which limits the re-use and adds to the cost of processing.
It is believed that the impaired conductivity of the nickel flake is caused by residual fatty acid lubricant or metallic soaps left on the flake after drying, and it has been found that when subjected after drying to annealing in a reducing atmosphere the flake can be made conductive, possibly by removing sufficient amounts of the residual lubricant. However, the temperatures required to make the flake conductive are too high, and result in sintering or agglomeration of the flake, thus destroying its usefulness. Lower temperatures at which sintering or agglomeration is prevented are not effective from the standpoint of conductivity.
It has been found that with the use of a suitable lubricant in the mobile liquid system as a substitute for fatty acid lubricants, conductive metal flake can be obtained, and it can be obtained in the dried condition without the need for an anneal step after the milling process. However, conductivity can be further improved by an annealing step. If an annealing cycle is used, it can be carried out at temperatures which do not present a sintering problem. Further, annealed flake can be made which has good environmental stability, e.g., decreased loss of conductivity due to exposure to moisture and/or elevated temperatures.
With respect to the processing operation, it has been found that with the lubricant of this invention, the deterioration of the liquid medium is, at least, reduced considerably, and the flake is produced with high production efficiency and reproducibility.
These and other advantages of the present invention will become apparent from the description which follows taken in consideration with the examples. In this connection, it is noted that although the present invention is described herein with particular reference to nickel, the same principles apply to other conductive metals and alloys, as will be evident to those skilled in the art.